The present invention relates to an image transfer device for a copier, printer, facsimile apparatus or similar image forming apparatus.
It is a common practice with an image forming apparatus to form an image on a photoconductive drum or similar image carrier, transfer the image from the drum to a sheet by an image transfer device, and then fix it on the sheet by a fixing device. The image transfer device may be implemented by a contact transfer scheme using, for example, a transfer belt or a non-contact transfer scheme corona discharger.
For example, Japanese Patent Laid-Open Publication No. 3-231274 (referred to as Prior Art 1 hereinafter) discloses an image transfer device using a transfer belt. The transfer belt is rotatable in contact with a photoconductive drum while a sheet is fed to a nip portion between the drum and the belt. As a transfer charge is applied from a power source to the belt via a transfer electrode contacting the belt, a toner image is transferred from the drum to a sheet being conveyed by the belt. The sheet carrying the toner image is separated from the drum and conveyed by the belt. At this instant, a controller measures a feedback current Ic flowing into the controller via a belt support roller and controls, based on the current Ic, a current Ir output from the power source such that a difference Ir-Ic has a constant value.
An image transfer device using a transfer roller is disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 3-158877 and 5-11645 (referred to as Prior Arts 2 and 3 hereinafter, respectively). Prior Art 2 executes constant current control by using two different target constant currents in order to adapt to changes in environment. Specifically, a detection electrode having a plurality of different load characteristics is held in contact with the surface of the transfer roller. When a sheet is conveyed, a voltage to be applied to the transfer roller is controlled on the basis of a current flowing through the detection electrode. With this kind of approach, it is possible to maintain optimal image transfer without regard to the varying resistance or irregular resistance of the transfer roller. On the other hand, Prior Art 2 causes the current-voltage characteristic of the transfer roller relative to a photoconductive element to converge to a given point on a preselected current-voltage curve. As a result, despite the varying resistance or irregular resistance of the transfer roller, an optimal transfer voltage is determined based on a current when a voltage is changed or on a voltage when a current is changed while a sheet is not conveyed.
A prerequisite with Prior Art 1 is that the control over the transfer current by the controller be extremely accurate and stable since the transfer belt is held in contact with the drum and applied with a bias, i.e., it is different from a corona discharger which does not contact the drum. If this prerequisite is satisfied, the device can achieve stable image transfer and sheet separation adaptive to the varying environment. However, Prior Art 1 has the following issues (1) to (5) yet to be solved.
(1) Generally, use made of transfer belts having electric resistances lying a predetermined range (.DELTA.10.sup.2). However, with the state-of-the-art-technologies, it i s extremely difficult to confine all the belts in the predetermined range of resistance. The only way available at the present stage of development to select acceptable belts out of all the products. This, however, lowers yield to a critical level, needs extra work for selection, and increases cost. The control over the transfer current by the controller promotes stable image transfer and sheet separation against a certain degree of irregularity in the resistance of the belt itself. However, the limited allowable level also results in the yield problem.
(2) Considering world topology, temperature and humidity greatly differ depending on the district and season. Even in offices, temperature and humidity are expected to noticeably differ, for example, in the early morning, depending on the district and season. Such a difference in temperature and humidity has noticeable influence on the condition of sheets (dry or wet) and, therefore, on the resistance of the transfer belt and other members directly contributing to image transfer. Further, the irregular resistance of the belt itself, as discussed in (1) above, aggravates the change in the resistance between the transfer electrode and the drum attributable to environment. With the control over the transfer current, therefore, it is difficult to ensure stable image transfer and sheet separation against conspicuous changes in environment combined with the irregular resistance of the belt.
(3) The electric resistance of a sheet noticeably changes during the course of image formation, depending on the image forming mode selected. This occurs when an image forming cycle is repeated a plurality of times with a single sheet. Typical of such an image forming mode are a duplex mode for forming an image on both sides of a single sheet, and a combination mode for forming different images on the same side of a single sheet. In the duplex mode, the image transfer ratio is lowered during the image transfer to the rear or second side of a sheet, compared to the image transfer to the front or first side, for the following reasons. To begin with, a sheet passed through the fixing device has had the moisture thereof reduced and, therefore, has increased in electric resistance. Also, such a sheet has lost flatness and has locally curled, often resulting in an air gap (between the drum and the sheet) just before the nip portion. Further, a discharge is apt to occur due to the air gap as the resistance of the sheet increases. As a result, for a given transfer current, the image transfer rate is apt to decrease more in the event of rear image transfer than in the event of front image transfer; this is particularly conspicuous when the transfer current is great. In the particular image forming modes mentioned above, since a toner image formed on a sheet by the first image forming cycle is fixed on the sheet by heat, the moisture of the sheet is less in the event of the second image forming cycle than in the event of the first image forming cycle. As a result, the resistance of the sheet greatly differs from the first image forming cycle to the second image forming cycle. It is, therefore, difficult to ensure stable image transfer and sheet separation with the control over the transfer current.
(4) The resistance of a sheet is dependent on the kind of a sheet also, i.e., thickness (without regard to the property of a sheet) and property (ordinary sheet or OHP (OverHead Projector) sheet). This obstructs stable image transfer and sheet separation for the saint reasons as stated in (1) and (2) above. This kind of change in the resistance of a sheet is further aggravated when (1), (2) and (3), as well as (5) which will be described, are combined.
(5) The electric resistance between the transfer electrode and the drum is further changed since the area over which the electrode and drum directly contact via a sheet changes with a change in the width of a sheet. The width of a sheet depends on the size of a sheet as measured in the axial direction of the drum. For example, the width of a sheet of A3 size oriented vertically long is 297 mm while the width of a sheet of B5 size in the same orientation is 182 mm. The change in resistance translates into a change in the adequate value of the previously mentioned Ir-Ic(=I.sub.out). The resulting relation between the sheet size and the adequate I.sub.out value is shown in FIG. 3. When I.sub.out is lower than the adequate value due to the size of a sheet, image transfer is defective. Also, when I.sub.out is higher than the adequate value, defective image transfer occurs since the toner is charged to opposite polarity by a discharge occurring in the gap between the sheet and the inlet side of the drum.
The change in the resistance between the transfer electrode and the drum stated above makes it difficult for the controller to promote stable image transfer and sheet separation for the same reasons as discussed in (1) and (2). This kind of change is further aggravated when (1), (2), (3) and (4) are combined.
On the other hand, Prior Art 2, using contact transfer means implemented as a transfer roller, is capable of effecting constant current control between the transfer roller and the detection electrode. However, the problem with Prior Art 2 is that the current to flow from the roller to the photoconductive element is not constant due to the influence of a difference the resistance of a sheet being conveyed in (difference thickness, difference in property between OHP sheets and ordinary sheets, and difference in moisture in the duplex mode or combination mode). As a result, the image transfer ability is affected by the condition of a sheet. Moreover, the detection electrode, contacting the transfer roller, is apt to suffer from smears due to toner particles and paper dust. In addition, when cleaning means is used to remove toner particles and paper dust from the transfer roller, defective cleaning occurs when the transfer roller wears due to the sliding contact thereof with the detection electrode.
Prior Art 3 does not maintain the transfer current constant under a condition wherein a sheet is conveyed. Hence, even in Prior Art 3, the image transfer ability is susceptible to the condition of a sheet due to tile difference in the resistance of a sheet being conveyed stated above.